The present invention relates generally to spacecraft, and more particularly, to spacecraft having a momentum wheel configurations and spacecraft employing same that prevents zero wheel speeds.
For a spacecraft having three-axis momentum wheel storage, the most basic momentum wheel arrangement is the use of three wheels that are disposed parallel to roll, pitch, and yaw axes of the spacecraft. While this provides three-axis momentum storage, it does not provide redundancy to protect against wheel failure.
To add redundancy, U.S. Pat. No. 3,999,729 teaches modifying the basic three orthogonal momentum wheel configuration and adds a fourth skew wheel that may be used as a replacement wheel in the event of failure of any of the three primary wheels. Another common wheel configuration is to arrange four wheels in a pyramidal configuration.
However, when the above described momentum wheel configurations are used to provide three-axis momentum storage with 4-for-3 redundancy, the momentum wheels may change rotational directions, thus causing the wheels to stop rotating for an instant.
These zero wheel speeds cause three problems. First, stiction (static friction) that occurs at zero wheel speed is uncertain, which may result in a spacecraft pointing error. Second, the distribution of bearing lubrication used to lubricate bearings of the momentum wheels may be less effective at low speeds, causing insufficient lubrication and reduced bearing life. Third, a tachometer used for sensing wheel speed is less effective at low speeds.
It would be therefore be advantageous to have momentum wheel configurations for use on a spacecraft that prevents zero wheel speeds, and spacecraft such momentum wheel configurations.
The present invention comprises a momentum wheel configuration that may be used on a spacecraft having a momentum bias that prevents zero wheel speeds. The present invention produces a momentum configuration that provides a momentum bias and three-axis momentum storage without causing the momentum wheels to experience zero wheel speeds.
More particularly, the present invention is a momentum wheel configuration for use with a spacecraft having a momentum bias and that comprises three or more momentum wheels. The momentum wheels are oriented such that their wheel axes are not parallel to each other. The momentum wheels are oriented such that their axial momentum vectors are not coplanar. None of the momentum wheels have their wheel axis parallel to the momentum bias direction of the spacecraft. No two of the momentum wheels are oriented such that their axial momentum vectors are coplanar with the momentum bias direction of the spacecraft.